I am not using the OnlyOffice Mail Server but am using an existing IMAP/SMTP server and assume this error is just indicative of that fact. Also when I click the link "Activate the email again" I don't see anything added to web.api.log nor do I see a (time) matching other log file, ie. no other log file seem to be updated at the time I try to resend the activation email.

Not knowing what else I rebooted the server and sure thing, email activation emails where delivered.

Hello jbakuwel!
I would advise you to look through logs because it's not clear where the reason is.
If it the issue repeats try to restart onlyofficeNotify service first of all just to be sure that the problem is connected with this service.

Would you suggest to restart the onlyOfficeNotify service periodically?